The present invention relates to an eccentric Archimedian screw pump of the rotary displacement type (sold under the trademark MOHNO PUMP). A prior art pump of this type is shown in FIG. 1 of the accompanying drawings, and, for example, in Japanese Patent Provisional Publication No. 51-13404 laid open on Feb. 2, 1976, claiming priority of German Patent Application No. P 24 29 340.1 filed on June 19, 1974.
In FIG. 1, the pump assembly includes a pumping section A, an inlet B, an outlet C and a drive Section D. The pumping section includes an elastic stator 2 having a pump cavity in the shape of a spiral female thread 3, and a pump rotor 4 in the shape of a spiral male thread which is in rotatable engagement within the thread 3. The drive section D includes a drive shaft 20 supported by bearings on the pump housing. The rotor 4 is rotatable eccentrically of the shaft 20, and connected with it through a pair of universal joints 50 and a connecting rod 51. This arrangement results in an increased length of the pump.
The foregoing arrangement has a number of disadvantages in addition to that of the increased length. The rotor motion is confined only by the inner wall of the thread 3, with the connecting rod 51 rotating out of parallel with the pump axis. As a result, the rotor 4 may rotate unstably and may swing outside the normal path, thereby fatiguing the inner wall of the thread 3.
In addition, the universal joint 50 which is generally located adjacent the inlet end of the stator cavity, increases the resistance to the flow of the pumped medium into the cavity. Further, long and solid objects in the medium may be caught around this joint.